The invention relates to a drive train of a hydraulic drive according to the preamble of Claim 1.
Drive trains of the generic type are often used in hydraulically driven, driveable operating machines, such as are known, for example, from DE 44 31 864 A1. In the case of such high-performance drives, the requirement is to reach high travelling speeds as well as a high level of tractive force with high levels of efficiency. Drives with two hydro-machines are increasingly being used for this purpose nowadays, both of which, as a rule, are adjustable. Examples for this are provided by DE 195 10 914 A1 and DE 43 07 616.
In this case, both hydro-motors are driven by a common hydro-pump and, as regards the circuit of the hydraulic fluid, are connected in parallel. The one hydro-motor can be adjusted to zero displacement to achieve a high travelling speed. This hydro-motor is then disengaged to avoid high flange losses, i.e. it goes to zero r.p.m. So that the driving power of said hydro-motor can be utilized again, it has to be engaged in the associated drive train by means of a coupling. If both hydro-motors are engaged at maximum displacement, the highest tractive force of the vehicle is thus available.
A disadvantage of the known drive train of the described type is that when the hydraulic machine or the hydro-motor is reconnected in the above-mentioned prior art and as a result the drive train is engaged, on account of the conventionally used synchronous couplings the hydraulic machine has first to be accelerated in order to reach the necessary synchronous speed for the coupling engagement. In this case, there is dependence on the operating pressure of the hydraulic drive and on the required speed, which makes control expensive and requires a considerable sensor system. This applies in an analogous manner to claw couplings.
When using synchronous couplings, the hydraulic machine must initially be brought to the speed of the transmission drive shaft, so that engagement, in particular in the case of claw couplings, is possible. When starting/running up to speed or accelerating, the displacement of an adjustable hydraulic machine can only be selected just so high that, prior to engagement, the necessary speed is not exceeded on the still load-free hydraulic machine. I.e. the displacement and consequently the torque of the load-free hydraulic machine at this point is precisely as high as is required by the inertia and/or friction of the hydraulic machine. If the displacement of the hydraulic machine, which is to be integrated in the drive train by way of a synchronous coupling, were to be adjusted, still in the load-free state, to the torque to be output/taken on, the speed set for the hydraulic machine in relation to the transmission or a consumer would be too high for the synchronous engagement. For fixed displacement machines with constant displacement, the speed control must be regulated by way of the hydraulic pressure and hydraulic fluid flow present in the motor.
If the load-free hydro-machine, to be synchronously engaged, has reached the speed of the consumer or transmission through corresponding adjustment of the displacement/pressure, engagement is able to take place, the torque output or to be taken on by the hydraulic machine in by far the majority of cases not corresponding to the expected power level of the hydraulic machine and the speed having to be readjusted commensurate with the torque. However, this cannot take place until after engagement, so that the speed of the hydraulic machine does not increase beyond that of the consumer or come to a standstill if the torque set is insufficient. Over and above this, the expensive and complicated control requires the engagement operation to be coordinated in time, said engagement operation not then always taking place smoothly in a manner that protects the materials. In a corresponding manner, this is often monitored by a complicated system of sensors.